1. Field of the Invention
The field of the invention is data processing, or, more specifically, methods, apparatus, and products for memory access alignment in a double data rate (‘DDR’) system.
2. Description of Related Art
The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely complicated devices. Today's computers are much more sophisticated than early systems such as the EDVAC. Computer systems typically include a combination of hardware and software components, application programs, operating systems, processors, buses, memory, input/output devices, and so on. As advances in semiconductor processing and computer architecture push the performance of the computer higher and higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago.
In a DDR system, DRAMs send a strobe (‘DQS’) coincident with multiple beats of data (‘DQ’). On reads, the memory controller chip is responsible for delaying the DQS to capture the DQ bits while meeting the setup and hold requirements at the capturing latches. On writes, the memory controller chip is responsible for delaying the DQS to be ideally centered within the DQ “eye” to ensure proper capture into the memory device. Before a DRAM device is used to store actual application data, the memory controller runs a training sequence where DQS and DQ arrival times at both the DRAM and memory controller are measured against reference clocks. These measurements are then used to adjust DQS and DQ delays to meet setup and hold times. As DRAM speeds increase, the likelihood of crossing a clock edge gets higher so the need to ensure alignment to the correct clock edge becomes greater.